To achieve a high degree of efficiency of work machines that are subject to the Clausius-Rankine cycle, such as e.g. steam turbines, it is necessary to carry out the steam/liquid phase transition of the working fluid in the power plant condenser at as low a condensation temperature as possible. At a temperature of 30° C., for example, water vapor pressures of about 40 mbar and less are reached.
The waste heat inevitably arising in the work machines must be led away in the condenser to the surrounding area. The practical transfer of the waste heat in power plants or in large industrial plants, in which similar cooling requirements exist, but no direct cooling in the condenser by means of water from rivers, lakes or the sea is possible, is achieved preferably through natural draft cooling towers or through fan-type coolers with sucking or blowing fans.
The release of the condensation heat of the working fluid to the surrounding area usually takes place in several steps: First condensation of the steam on the outer surface of the condenser pipes, conduction of heat through the pipe material to the pipe interior, heat transfer through forced convection from the pipe interior to the cooling medium (usually cooling water) flowing through the pipes and finally thermal emission from the cooling medium to the surrounding area.
With operation of the power plant at high electrical output, a correspondingly large coolant flow is necessary, which has to be pumped through the condenser pipes. If, for example, a power plant has an electrical output of 1000 MWe, with a good power efficiency of 40%, a waste heat flow of 1500 MWth takes place as heat loss flow at low temperatures, which has to be transferred in the condenser from steam to the cooling medium and finally to the surrounding area. If the condenser is cooled with cooling water from a river, lake or the sea, the cooling water flow necessary therefor is about 36 metric tons/sec, if the cooling water must not be warmed up by more than 10 K. Since such large quantities of cooling water are often not available, the heat transfer from the cooling medium to the ambient air takes place in cooling towers.
In wet cooling towers, the heated cooling water originating from the process flows for the most part as film flow downwards from above on a wet surface towards an upward flowing air current. The air current is generated by fans or in high towers by natural chimney effect or by a combination of both. The heat transfer from the cooling medium to the upwards flowing air takes place for the most part through its evaporation in the air current, and since the evaporation heat of the cooling water is very large, the necessary water requirement from the surrounding area can be reduced by up to two orders of magnitude compared with the requirement with direct cooling from river, lake or the sea. Thus an air current loaded with water vapor flows upward out of the cooling tower.
However, the quantity of air that flows upwards through the cooling tower in order to convey away the waste heat of the plant to be cooled is also very large. In the above-mentioned example with the waste heat of 1500 MWth, the requirement in air is 20 to 30 metric tons/sec., depending upon prevailing meteorological conditions.
Another cooling concept consists in dry/wet hybrid cooling towers, which are used predominantly for elimination, or at least reduction, of the visible plume of humid air of the wet cooling towers. Here, too, the required quantity of air is very large, usually twice as large as the quantity of air of a wet cooling tower with an equal performance rating.
At sites where the quantity of water required for the evaporation cooling is not available, dry coolers are used. The quantity of air that is necessary for the convective heat transfer in such coolers is however about six times the requirement of the corresponding wet cooling towers.
The quantity of air is therefore an important parameter for the design and operation of all kinds of cooling towers, and is decisive for their physical size and costs, among other things with respect to the built-over area which such plants require. The conveying of the large air quantities is a challenging problem for the development of the towers. Several influences play a big role here, above all those of the meteorological constraints, which are closely linked to the functioning of the tower. Not only must the towers reliably absorb the thermal load in a wide range of meteorologically contingent temperatures and wind velocities and be able to release it into the atmosphere, they must also successfully fulfil this task in special situations such as heavy icing in winter or during storms.
With the conveying of the required quantity of air through the towers, a sufficiently high flue in the case of natural draft, and a correspondingly dimensioned large number of fans in the case of forced ventilation, must be operated, which are able to overcome the unavoidable flow pressure losses in the tower. The pressure losses are thus a further decisive parameter in the dimensioning of the towers, since they determine the required height of the towers with natural draft as well as the capacity of the fans when the tower or tower section is forced-air cooled.